1. Field of the Invention
This invention relates in general to a resonant testing method and, more particularly to use of temperature compensation of measurements made in a variable temperature location such as a manufacturing operation. Temperatures may vary within a day, or especially, as a part cools after processing. Temperature compensation may be required whenever resonantfrequencies are precisely measured. In particular when resonant ultrasound spectroscopy is used, as illustrated in U.S. Pat. Nos. 5,062,296; 5,408,880; 5,631,423; 5,641,905; 5,425,272; and 5,495,763, temperature compensation may be desired.
2. The Prior Art
Testing of objects by use of sound (including ultrasound) and vibrations is well known. The prior art is extensive and encompasses many types of non-destructive testing. Resonant sound and ultrasound have been used for testing purposes as described in U.S. Pat. Nos. 5,062,296; 4,976,148; and 5,355,731 which are incorporated herein by reference except for their incorporation by reference of other information.
Resonant frequencies of any stiff object, such as a manufactured part or a test specimen, made of metal or ceramic, are fundamentally determined by its dimensions and elastic constants. This observation has been used since the 19th century to detect the presence of defects. For example, railroad engineers are said to have struck wheels and determined the presence of a crack by the change in the audible resonant tone (resonance). The resonance of a cracked wheel has a distinctly lower frequency than that of a good wheel. Since the 1950s, commercial instruments have been available that use this phenomena to provide an impulse to a part and measure the resonant frequency response. These instruments have been used to detect defects in manufactured parts and to measure the elastic constants of test specimens. Generally their accuracy is on the order of 1%, so that they cannot detect the effects of temperature variations on the resonant frequencies. Their relatively poor accuracy also limits their ability to detect small defects.
In the 1980s, Resonant Ultrasound Spectroscopy (RUS--U.S. Pat. No. 5,062,296) was developed and later applied to defect detection and measurement of elastic constants. RUS instruments can make measurements to an accuracy of 0.01%. At this accuracy the effects of temperature can be noticed. This allows detection of much smaller defects, but it also means that the temperature variations can mask the effects of the small defects or small changes in elastic constants. U.S. Pat. Nos. 5,631,423; 5,408,880; 5,495,763; 5,641,905; and 5,425,272 are examples of this method and apparatus. Each of these patents is incorporated by reference except for their incorporation by reference of other material.